Bioreactors for wastewater treatment are classified as dispersed-growth reactors and biofilm reactors.
In the dispersed-growth reactors, such as the activated sludge process, the biomass exists usually as suspended flocs. In order to reduce reactor volume, a high biomass concentration must be maintained in the reactor, in order to obtain this, biomass recycle from a settling tank is usually employed. This type of reactor is not usually used for anaerobic treatment of low-strength wastewater because of the relatively short solids retention time that results.
Biofilm reactors are distinctive in their ability to achieve the high solids retention time needed for efficient anaerobic treatment because the biomass is retained in the reactor by attaching to the media surface. Such reactors include packed bed reactors having fixed media and fluidized bed reactors having fluidized media.
In the fluidized bed reactors, fluid flows at sufficient velocity, generally in an upflow manner, through a bed of particulate matter such as sand or granular activated carbon such that the particles are lifted and remain suspended in the fluid. Here, the downward force of gravity on the particles is just balanced by the upward velocity of the fluid flowing around the particles. Fluidized bed reactors have seen many environmental applications, particularly for removal of nitrate and perchlorate from wastewaters. Here, microorganisms grow as dense biofilms attached to the fluidized particles, permitting the maintenance of a large biomass concentration within the reactor as needed for high efficiency of treatment at short detention times on the order of minutes rather than hours that may be required in other types of reactors.
On the other hand, the dispersed-growth reactors have been equipped with membranes for the maintenance of a high concentration of microorganisms within the reactor, leading to high treatment efficiency at short detention times. Either microfiltration or ultra-filtration membranes have commonly been used for this purpose, permitting treated water to pass through the membrane while maintaining particulate matter of the size of microorganisms—about 1 μm in size—to remain within the reactor. Because of the exclusion of such small particulate material from the reactor effluent, the effluent tends to be quite clean and can be disinfected readily, reaching a quality when treating municipal wastewaters that is adequate for safe use for irrigation of food crops. This represents an advantage over a traditional wastewater treatment system, which to meet similar effluent quality standards generally requires in addition to the biological reactor, a final settling tank to remove larger particulate matter and a multi-media filtration step to remove smaller ones of bacterial size. The advantage of the membrane bioreactor in this case is a treatment system that uses much less overall treatment system volume and requires a much smaller space, that is, it has a smaller footprint.
However, membrane fouling has been a serious problem affecting system performance in membrane bioreactors. Membrane fouling is caused by deposition of foulant materials on membrane surface and/or adsorption of them into membrane pore matrix. Membrane fouling decreases membrane performance due to increasing hydraulic resistance across the membrane, thereby increasing capital and operational costs subsequently. There have been many different approaches to reduce membrane fouling in membrane bioreactors. The common approach currently used in membrane bioreactor systems is the introduction near the membranes of high turbulence and cross-flow created by the introduction of air or recycled biogas below the membranes. While reasonably effective, this has a high energy cost. In addition, back pulsing of liquid or gas through the membrane and periodic cleaning are common practices, which also require high cost.
Aerobic treatment is the most common biological treatment system for wastewaters, but requires significant energy for air injection. Anaerobic processes, however, operate without air or oxygen introduction, and instead produce useful energy in the form of biogas. A specific need of anaerobic treatment is a long solids retention time (SRT) to prevent washout from the reactor of slow-growing methane-forming anaerobic bacteria. Membrane bioreactors can be advantageous for anaerobic treatment of dilute wastewaters such as domestic sewage in that the membranes prevent washout of anaerobic bacteria and thus can provide the long SRT needed, while operating at short hydraulic detention times as required to reduce reactor size and cost. The higher current energy requirement with membrane treatment, however, tends to offset the advantage.
Even though much research has been conducted in an effort to reduce this energy requirement, the needs to reduce the energy requirements further still exists.